Lighted indicators for aircraft cockpits have typically been illuminated with incandescent bulbs. Due to the short life of incandescent lamps, these indicators must include mechanisms that allow for removal of burned out lamps. Together with making the device more complex and costly, these mechanisms often fail or are damaged in service. Additional cost is reflected in high airline maintenance activity as a result of replacing burned out lamps and damaged indicators and associated airplane downtime. In addition to these reliability issues, there are other drawbacks with incandescent indicators. The light and color output emitted from incandescent lamps varies proportionally with input voltage applied. Under certain operating conditions, when the input voltage drops low enough, the brightness of the indicator can drop so low that it becomes difficult to recognize or see. In addition the brightness change the color that an incandescent lamp emits shifts towards the red spectrum as the voltage is reduced. In flight deck aviation applications the color that an indicator emits indicates to the flight crew what type of action is expected. For example red indicates to the pilot a condition that requires immediate action, where amber indicates a cautionary condition and white is used to indicate a normal operating condition. Thus with incandescent light indicators, the perceived color of the indicator can change with varying inputs voltages. This color ambiguity is particularly acute with indicators that display white or amber.
Recently indicators that use LEDs have been introduced in aircraft applications. If applied correctly LEDs can achieve significantly longer operating lifetimes than incandescent lamps resulting in little or no aircraft maintenance. Another advantage of LEDs is that their color spectrum remains essentially constant as their power level is varied. The color emitted from an LED indicator therefore is very consistent with input power levels, thus providing the flight crew with a very unambiguous color indications. However this color stability presents a difficulty when mixing LED indicators with incandescent indicators since the color of the two types can only match at a single unique input power or voltage level. For example the color of an amber LED indicator essentially matches the color of a white incandescent indicator when operating at lower brightness operating levels thus introducing the potential of miss-alerting the flight crew.
While LEDs can exhibit many positive characteristics, there are design constraints that must be recognized in order to realize a longer life, lower cost LED driven indicator. LEDs are voltage devices and therefore require a means to limit the current supplied as the voltage is increased above their intrinsic forward voltage drop. When operating LEDs in series strings, as is often done, the sum of the forward voltage drops can exceed the input voltage available. In this circumstance a more complicated power supply is required to raise the voltage supplied to the LEDs to the level necessary to turn on the LEDs. In addition to the increased complexity and cost of this voltage conditioning circuitry, it often results in excess heat and high electromagnetic emissions. Although LEDs potentially have extremely long life expectancy, failure to regulate the power adequately or allowing the temperature of the semiconductor junction of the LED to exceed the limit of the material, can cause the LED to fail prematurely. Operating the LED under excessive heat loads will also reduced the expected lifetime and overall light output.
The physical outline and electrical interface of flight deck aviation indicators has varied significantly from one aircraft design to another. This often precludes the ability to interchange indicators from one airplane model to another. This results in higher manufacturing cost due to the increased number part numbers to inventory, which also translates to higher airline hardware maintenance inventory costs.
Therefore, there is an unmet need for a cheaper-to-build, more reliable LED indicator.